In tunnel furnaces commonly a closed cart train is moved through the tunnel furnace, the steel structures of the furnace carts being protected against the burning channel by a superstructure of refractory and insulating materials. Since large pressure differences prevail in the burning channel due to the different temperatures and draught conditions, the under-cart channel disposed below the furnace carts must be sealed-off in order to protect the steel parts.
For sealing-off the under-cart channel disposed below the furnace carts against the burning channel, there are suggestions in the prior art to guide sand grooves laterally along the furnace carts, into which grooves the continuous sheet metal plates fastened to the lateral sides of the steel structures of the furnace carts immerge as sealing aprons. At the cart ends adjoining each other there are arranged labyrinth-like seals having refractory gasket cords.
However, the said known sealings are insufficient. Since it is required that cooling air for cooling the steel structure including furnace cart wheels with bearings be moved through the under-cart channel, there are draught and pressure conditions prevailing in the under-cart channel which are different from those in the burning channel above the furnace carts.
Tunnel furnaces in modern plants have channel widths of up to 10 meters, and the sealings of the furnace carts require higher precision than can be achieved by simple means. Thereby leaks occur between the burning channel above the furnace carts and the under-cart channel. Hot gases then escape from the burning zone and travel into the cooling channel (under-cart channel), where they endanger the steel structure and in particular the furnace cart wheels and bearings. On the other hand, cooling air escapes uncontrolledly from the cooling channel below the furnace carts, traveling into the burning channel above the furnace carts and thus damaging the material to be burnt.
From the European Patent Application No. 86 115 658.6 (publication number 0 227 927) it is known to divide the under-cart channel into separate small chambers, through which cooling pipes are led from the outside in order to cool the air in said chambers. Said cooling pipes are disposed below the furnace carts, at a distance of about 0.5 m from the steel structures of the tunnel furnace carts to be cooled. Since the furnace cart structure that is to be cooled is at the highest point of the cooling chambers, it is clear that at that point also the space and the air are the hottest.
Therefore it appears not very useful to cool the air at a distance of about 0.5 m below the undersides of the furnace carts, because as a result of its lower specific weight the hot air travels upward, and the desired natural circulation cannot be achieved to a sufficient extent. It would rather be necessary to install artificial circulation means, something that becomes uneconomical in view of up to 100 furnace carts per tunnel furnace and 2 chambers per furnace cart.